Atmospheric corrosion-resistant steel, also called weather-resistant steel, refers to the low-alloy structural steel having a protective rust layer of atmospheric corrosion resistance, which can be used to make vehicles, bridges, towers, containers and other steel structures. Compared with plain carbon steel, weather-resistant steel has a more excellent corrosion-resistant performance in atmosphere; compared with stainless steel, weather-resistant steel only contains trace amounts of alloy elements like P, Cu, Cr, Ni, Mo, Nb, V, Ti, etc., the total amount of which accounts for only a couple of percentage points (in the case of stainless steel, it accounts for a dozen of percentage points), so its price is relatively lower.
The atmospheric corrosion-resistant steel types frequently used in recent years are 09CuPTiRE of 295 MPa-grade, 09CuPCrNi of 345 MPa-grade and Q450NQR1 of 450 MPa. With the development of national economy, requirements are increasing on vehicle weight reduction, speed acceleration, freight volume increase, service life extension, logistics cost reduction, etc., above-mentioned steel types can no longer meet the requirements, so developing high-strength, highly corrosion-resistant and low-cost atmospheric corrosion-resistant steel presents important practical value and economic significance. At present, many patents have been applied for on high-strength atmospheric corrosion-resistant steel and its manufacturing method both at home and abroad, wherein the atmospheric corrosion-resistant steel having strength of 700 MPa-grade, the (Nb, V, Ti and Mo) multi-microalloying technology is generally used to improve its comprehensive mechanical property through refined crystalline strengthening and precipitation strengthening.
Chinese Patent 200610030713.8 discloses an atmospheric corrosion-resistant steel with a yield strength of 700 MPa-grade and its manufacturing method, by which the atmospheric corrosion-resistant steel sheet is manufactured with the chemical composition as follows: C 0.05˜0.1%, Si≦0.5%, Mn 0.8˜1.6%, P≦0.02%, S≦0.01%, Al 0.01˜0.05%, Cr 0.4˜0.8%, Ni 0.12˜0.4%, Cu 0.2˜0.55%, Ca 0.001˜0.006% and N 0.001˜0.006%, and at least two elements selected from Nb, Ti and Mo having a content of Nb≦0.07%, Ti≦0.18% and Mo≦0.35%, and balance being Fe and inevitable impurities. The steel sheet thus manufactured has yield strength of 700 MPa or above, a tensile strength of 750 MPa or above and an elongation of 15% or above.
Chinese Patent 201010246778.2 discloses a non-quenched and tempered (NQT) low-cost and high-strength weather-resistant steel with a yield strength of 700 MPa-grade and its manufacturing method, by which the weather-resistant steel sheet is manufactured with the chemical composition as follows: C 0.05˜0.1%, Si≦0.15%, Mn 1.5˜2%, P≦0.015%, S≦0.01%, Cr 0.3˜0.8%, Ni 0.15˜0.4%, Cu 0.2˜0.4%, Nb 0.02˜0.08%, Ti≦0.09˜0.15% and N≦0.005%, and balance being Fe and inevitable impurities. The steel sheet thus manufactured has yield strength of 700 MPa or above, a tensile strength of 800 MPa or above and an elongation of 18% or above.
Chinese Patent 200610125125.2 discloses an extra-high-strength atmospheric corrosion-resistant steel and its manufacturing method, by which the atmospheric corrosion-resistant steel sheet is manufactured with the chemical composition as follows: C 0.01˜0.07%, Si 0.25˜0.5%, Mn 1.6˜2%, P≦0.018%, S≦0.008%, Al≦0.035%, Cr 0.4˜0.75%, Ni 0.25˜0.6%, Cu 0.2˜0.5%, Nb 0.03˜0.08%, Ti≦0.02%, Mo 0.1˜0.4% and B 0.0005˜0.003%, and balance being Fe and inevitable impurities. The steel sheet thus manufactured has yield strength of 700 MPa or above, a tensile strength of 750 MPa or above and an elongation of 10% or above.
The microalloying technology and the traditional hot rolling process have been employed in the manufacture of all above-mentioned types of atmospheric corrosion-resistant steel having a high-strength of 700 MPa-grade, which is composed of such alloy elements like Nb, V, Ti and Mo in their component systems. By the traditional hot rolling process, i.e., continuous casting+reheating and thermal insulation of the casting slab+rough rolling+finishing rolling+cooling+coiling, firstly the casting slab of about 200 mm in thickness is produced by continuous casting, next it is subjected to reheating and thermal insulation, then to rough rolling and finishing rolling to obtain the steel strip generally greater than 2 mm in thickness, and finally the steel strip is subjected to laminar cooling and coiling to complete the entire hot rolling manufacturing process. If a steel strip less than 2 mm in thickness is to be manufactured, generally the hot-rolled steel strip needs to be subjected to further cold rolling and subsequent annealing. However, there are the following main problems existing in the traditional process manufacturing microalloyed high-strength atmospheric corrosion-resistant steel,                (1) The manufacturing cost is high caused by long process flow, high energy consumption, multiple unit equipment, high infrastructure construction cost;        (2) Given that the atmospheric corrosion-resistant steel contains relatively high contents of P, Cu and other easy-segregation elements which can improve the atmospheric corrosion-resistant performance of the steel strip, the traditional process, due to the low solidification and cooling rates of the casting slab, may easily cause the macroscopic segregation of P, Cu and other elements, the anisotropy, macroscopic cracking and further low yield of the casting slab;        (3) The weather-resistant performance of the atmospheric corrosion-resistant steel is mainly determined by the combined action of P and Cu. Due to its easy segregation characteristic in the traditional process, P is frequently omitted from the composition design of the high-strength atmospheric corrosion-resistant steel manufactured by the traditional process, and its content is controlled within the level of an impurity element, i.e., usually ≦0.025%; the additive amount of Cu is in the range of 0.2˜0.55%, usually equal to the lower limit in the actual manufacturing practice. The result of said practice is the low weather-resistant performance of the steel strip;        (4) In the traditional process, the microalloy elements cannot be kept in the form of solid solution in the hot rolling process and usually go through partial precipitation and lead to the increase of steel strength, which thus significantly increases the rolling load, raises energy consumption and roller consumption, causes significant damage to equipment and therefore limits the thickness range of the high-strength hot-rolled weather-resistant product which can be economically and practically manufactured (i.e., usually ≧2 mm). Continuously subjecting the traditional hot-rolled product to cold rolling can further reduce the thickness of the steel strip. However, the high strength of the hot-rolled steel strip may also result in difficulties in cold rolling, in that the high cold rolling load imposes a relatively high requirement on equipment and causes relatively significant damages and that the second phase segregated from the alloy elements in the hot-rolled product significantly increases the recrystallization annealing temperature of the cold-rolled steel strip;        (5) When manufacturing a high-strength product containing microalloy elements by the traditional process, the principle of refining austenite grains through deformation is usually employed, thus, the initial rolling temperature of finishing rolling is usually lower than 950° C., and its final rolling temperature is around 850° C. Therefore, when rolling under a relatively low temperature and combined with the increase of deformation with the progress of the rolling process, the strength of the steel strip are significantly increased, thus, the difficulty and consumption of hot rolling are significantly increased.        
If the thin slab continuous casting and rolling process is employed to manufacture the microalloyed high-strength atmospheric corrosion-resistant steel, such disadvantages of the traditional process may be overcome to a certain extent. The thin slab continuous casting and rolling process, i.e., continuous casting+thermal insulation and soaking of the casting slab+thermal continuous rolling+cooling+coiling, distinguishes itself from the traditional process mainly in the following aspects. Firstly, in the case of the thin slab continuous casting and rolling process, the thickness of the casting slab is significantly reduced to 50˜90 mm. Since the casting slab is thin, the casting slab only needs to go through 1˜2 passes of rough rolling (the thickness of the casting slab ranging between 70 mm and 90 mm) or does not have to go through any rough rolling (the thickness of the casting slab less than 50 mm). In the case of the traditional process, on the contrary, the casting slab needs to repeatedly go through multiple passes of rolling before being thinned to the specification required before finishing rolling. Secondly, in the case of the thin slab continuous casting and rolling process, the casting slab directly enters the soaking furnace without cooling for soaking and thermal insulation or for small amount of temperature compensation), thus, the thin slab continuous casting and rolling process significantly shortens the process flow, reduces energy consumption, saves investment and reduces the manufacturing cost. Thirdly, in the case of the thin slab continuous casting and rolling process, the solidification and cooling rates of the casting slab are accelerated, which can reduce the macroscopic segregation of the easy-segregation elements to a certain extent and thus reduce product defects and improve the yield of products. Because of this, the composition design of the microalloyed high-strength atmospheric corrosion-resistant steel manufactured by the thin slab continuous casting and rolling process has widened the range of content of increasing corrosion-resistant elements P and Cu, which is favorable for improving the weather-resistant performance of the steel.
Chinese Patent 200610123458.1 discloses the method for manufacturing a 700 MPa-grade high-strength weather-resistant steel adopting the Ti microalloying technology based on the thin slab continuous casting and rolling process, by which the atmospheric corrosion-resistant steel sheet is manufactured with the chemical composition as follows: C 0.03˜0.07%, Si 0.3˜0.5%, Mn 1.2˜1.5%, P≦0.04%, S≦0.008%, Al 0.025˜0.05%, Cr 0.3˜0.7%, Ni 0.15˜0.35%, Cu 0.2˜0.5%, Ti 0.08˜0.14% and N≦0.008%, and balance being Fe and inevitable impurities. The steel sheet thus manufactured has a yield strength of 700 MPa or above, a tensile strength of 775 MPa or above and an elongation of 21% or above. In the patent, P is controlled as an impurity element, with its content controlled to be 0.04% or below, which means that its content has been widened in comparison with the content of 0.025% or below of the traditional process.
Chinese Patent 200610035800.2 discloses the method for manufacturing a 700 MPa-grade V—N microalloyed atmospheric corrosion-resistant steel based on the thin slab continuous casting and rolling process, by which the atmospheric corrosion-resistant steel sheet is manufactured with the chemical composition as follows: C≦0.08%, Si 0.25˜0.75%, Mn 0.8˜2%, P≦0.07˜0.15%, S≦0.04%, Cr 0.3˜1.25%, Ni≦0.65%, Cu 0.25˜0.6%, V 0.05˜0.2% and N 0.015˜0.03%, and balance being Fe and inevitable impurities. The steel sheet thus manufactured has a yield strength of 700 MPa or above, a tensile strength of 785 MPa or above, and an elongation of 21% or above. In the patent, P is controlled as a highly corrosion-resistant element, with its content controlled to be in the range of 0.07˜0.15%; the content of Cu is in the range of 0.25˜0.6%, which means that its upper and lower limits are respectively higher than those of the content of Cu in the traditional process (i.e., 0.2˜0.55%).
The thin slab continuous casting and rolling process enjoys said advantages in the manufacture of microalloyed high-strength atmospheric corrosion-resistant steel, however, some problems existing in the traditional process still persist in the thin slab continuous casting and rolling process. For example, the microalloy elements cannot be kept in the form of solid solution in the hot rolling process and usually go through partial precipitation and lead to the improvement of steel strength, which thus significantly increases the rolling load, increases energy consumption and roller consumption, therefore limits the thickness range of the high-strength hot-rolled weather-resistant product which can be economically and practically manufactured (i.e., thickness of 1.5 mm or above). See details in Patents 200610123458.1, 200610035800.2 and 200710031548.2.
The continuous strip casting technology is a cutting-edge technology in metallurgy and material research fields, and its emergence has brought about a revolution in the steel industry and changed the manufacturing process of the steel strip in the traditional metallurgical industry. Besides integrating such procedures like continuous casting, rolling and even thermal treatment makes one-stop production of the thin steel strip from the produced thin steel slab through only one pass of online rolling, it also significantly simplifies the manufacturing procedure, shortens the manufacturing cycle (with a process line only 50 m in length), correspondingly saves equipment investment and greatly reduces the product cost.
The twin-roller continuous strip casting process is a primary form of the continuous strip casting process, and also the only industrialized form of the continuous strip casting process. In the twin-roller continuous strip casting process, the molten steel is introduced from the steel ladle through the long nozzle, tundish and submersed nozzle to the molten pool formed by a pair of relatively rotating and internally water-cooling casting rollers and the side dams, and forms solidified shells on the mobile roller surface which then assemble in the clearance between the two casting rollers, thus forming the cast strip pulled out downward from the roller clearance. After that, the casting strip is delivered to the roller bed through the swinging guide plate and pinch roller, and then goes from the online hot rolling mill through the spray cooling and flying shear to the coiling machine until the manufacture of continuous strip casting products is completed.
So far there has been no report on employing the continuous strip casting technology to manufacture the microalloyed high-strength atmospheric corrosion-resistant steel, and such approach may present the following advantages:                (1) The continuous strip casting process eliminated several complex processes like slab heating, multi-pass repeated hot rolling, etc., and directly provides one-pass online hot rolling for the thin cast strip, which significantly reduces the manufacturing cost;        (2) The cast strip produced by the continuous strip casting process usually has a thickness of 1˜5 mm, and can have an expected product thickness through online hot rolling (i.e., usually 1˜3 mm), and the manufacture of low-thickness products does not need the cold rolling process;        (3) When the continuous strip casting process is employed to manufacture low-carbon microalloyed steel, such added alloy elements like Nb, V, Ti and Mo mainly exist in the form of solid solution in the hot rolling process, so the steel strip has a relatively low strength, the reduction rate of hot rolling by a single-standard hot rolling mill can reach as high as 30˜50%, and the thinning efficiency of the steel strip is relatively high;        (4) When the continuous strip casting process is employed to manufacture low-carbon microalloyed steel, the high-temperature cast strip is directly subjected to hot rolling, and such added alloy elements like Nb, V, Ti and Mo primarily exist in the form of solid solution in the process, so the utilization rate of these alloy elements can be improved. In comparison, in the traditional process, the precipitation of these alloy elements occurs in the cooling process of the slab, and an inadequate redissolution of these alloy elements will occur when the slab is reheated, as a result of which the utilization rate of these alloy elements is reduced.        
However, the atmospheric corrosion-resistant steel is a type of relatively special products. It is usually required to have a superior strength and plasticity matching, so even on products with a relatively high strength grade, a relatively high requirement is imposed with respect to their elongation, otherwise the requirements of the forming process can not be met. When using the products which are manufactured by the continuous strip casting process and contain such microalloy elements like Nb, V, Ti and Mo, the inhibitory action of these microalloy elements to the recrystallization of the hot-rolled austenite may retain the inhomogeneity of the steel strip's coarse austenite grains. As a result, the microstructure of the final product produced through the phase change of the inhomogeneous coarse austenite also tends to be inhomogeneous, as a result of which the elongation of the product is relatively low.
International Patents WO 2008137898, WO 2008137899 and WO 2008137900 as well as Chinese Patents 200880023157.9, 200880023167.2 and 200880023586.6 disclose the method for manufacturing a microalloyed steel strip of 0.3˜3 mm in thickness by adopting the continuous strip casting and rolling process, wherein the steel strip is manufactured with the chemical composition as follows: C<0.25%, Mn 0.20˜2.0%, Si 0.05˜0.50% and Al<0.01%, and at least one element selected from Nb, V and Mo, having a content of Nb 0.01˜0.20%, V 0.01˜0.20% and Mo 0.05˜0.50%. Under the process conditions of the hot rolling reduction rate of 20˜40% and the coiling temperature of 700° C. or below, the microstructure of the hot-rolled strip is bainite+acicular ferrite. As disclosed in these patents, alloy elements are added to inhibit the recrystallization of the austenite after hot rolling, retain the coarse characteristic of the continuous strip casting austenite grains for hardenability improvement, and thus obtain the microstructure of bainite+acicular ferrite at room temperature. Moreover, the disclosure does not provide the temperature range adopted by the hot rolling, however, in papers related to these patents (C. R. Killmore, etc. Development of Ultra-thin Cast Strip Products by the CASTRIP® Process. AIS Tech, Indianapolis, Ind., USA, May 7˜10, 2007), the hot rolling temperature adopted is reported as 950° C.
The continuous strip casting low-carbon microalloyed steel product manufactured by this method has a relatively high strength, and can reach yield strength of 650 MPa and a tensile strength of 750 MPa within the range of said composition. However, the key problem is the low elongation of the product, the cause of which is explained below. The cast strip produced by the continuous strip casting process usually has coarse and extremely inhomogeneous austenite grains (from as low as dozens of microns to as high as 700˜800 microns or even in the magnitude of millimeter; the hot rolling reduction rate of the continuous strip casting process usually does not exceed 50%, and the effect of refining austenite grains through deformation is thus very insignificant. If these austenite grains are not refined through recrystallization, the inhomogeneous coarse austenite won't be effectively improved after hot rolling, and the bainite+acicular ferrite structure produced through the phase transformation of the inhomogeneous coarse austenite will also be extremely inhomogeneous, as a result of which the elongation of the product will be relatively low.
In order to improve the strength and plasticity matching of the continuous strip casting microalloyed steel, the Chinese Patent 02825466.X proposes the method for manufacturing a microalloyed steel strip 1˜6 mm in thickness adopting the continuous strip casting and rolling process, by which the microalloyed steel has a chemical composition as follows: C 0.02˜0.20%, Mn 0.1˜1.6%, Si 0.02˜2.0%, Al<0.05%, S<0.03%, P<0.1%, Cr 0.01˜1.5%, Ni 0.01˜0.5%, Mo<0.5%, N 0.003˜0.012%, Ti<0.03%, V<0.10%, Nb<0.035% and B<0.005%, and balance being Fe and inevitable impurities. The hot rolling of the cast strip is conducted corresponding to the austenite zone, austenite-ferrite two-phase zone or ferrite zone within the temperature range of 1,150-(Ar1-100)° C., with a hot rolling reduction rate of 15˜80%. In the method, an online heating system (with the heating temperature ranging between 670° C. and 1,150° C.) is designed to be set behind the continuous strip casting and rolling mill, the purpose of which is the complete recrystallization of the strip hot rolled in different phase zones occurs after thermal insulation for a certain period, so as to achieve a superior strength and plasticity matching for the steel strip.
When employing such method to manufacture the continuous strip casting low-carbon microalloyed steel product, the steel strip produced can indeed be endowed with a superior strength and plasticity matching. For example, for the steel strip which has a chemical composition including C 0.048%, Mn 0.73%, Si 0.28%, Cr 0.07%, Ni 0.07%, Cu 0.18%, Ti 0.01%, Mo 0.02%, S 0.002%, P 0.008%, Al 0.005% and N 0.0065%, its yield strength, tensile strength and elongation are respectively 260 MPa, 365 MPa and 28%. However, employing such method for manufacture requires that an online heating system be added during product line design, and that the heating furnace must be of sufficient length to ensure heating uniformity as the length of heating time is determined by both casting speed and heating furnace length. In this case, it not only increases investment cost, but also significantly increases the area occupied by the continuous strip casting and rolling production line and reduces the advantages of the production line.
In conclusion, when employing the continuous strip casting process to manufacture the microalloyed high-strength atmospheric corrosion-resistant steel with a superior strength and plasticity matching, given the low thickness of the cast strip, it's impossible to refine austenite grains through deformation, so the key lies in how to properly refine austenite grains through recrystallization, endow the product with a refined and homogeneous microstructure and thus achieve a superior strength and plasticity matching.